The present invention relates to a technical field concerning optical information processing.
In recent years, optical disc drives capable of recording information on and reproducing the information from CD-R/RW (Compact Disc-Recordable/Rewritable), DVD-R/RW (Digital Versatile Disc-Recordable/Rewritable), or the like have become widely spread. In the optical disc drive, in order to record the information, a light beam is irradiated onto an optical disc by an optical pick-up device contained in the optical disc drive. Then, in order to read the information, the light beam reflected from the optical disc is detected by the optical pick-up device.
In order to perform stable recording and reproduction processing during the recording and reproduction of the information by the optical pick-up device, it is necessary to accurately control the light amount of the light beam irradiated onto the optical disc. For this reason, the optical pick-up device has a unit for detecting the light amount of the light beam emitted from a laser diode. By feedbacking the detected light amount to the laser diode, the light amount of the light beam irradiated onto the optical disc is accurately controlled.
In order to detect the light amount of the light beam emitted from the laser diode, it is a common practice to provide a light reception element that receives a part of the light beam emitted forward from the laser diode. Herein, the light reception element that receives the light beam emitted forward from the chip of the laser diode will be referred to as a “front monitor”.
Various units for guiding the light beam from the laser diode to the front monitor are known.
Among these, an optical pick-up device disclosed in FIG. 1 of JP-A-2001-184709, for example, has a configuration in which a light beam is emitted from a laser diode, and a part of the emitted light beam outside the effective diameter of an objective lens is detected by a front monitor.
In this configuration, the light beam outside the effective diameter of the objective lens is monitored. Thus, it is possible to avoid a problem that a light amount detected by the front monitor is greatly varied. This problem occurs due to interference between the light beam emitted from the laser diode and a light beam reflected from the optical disc on the detection surface of the front monitor.
An optical pick-up device disclosed in FIG. 1 of JP-A-2003-77170, for example, has a configuration in which a diffraction element is arranged between an objective lens and a laser diode, so that the light amount of the light beam outside the effective diameter of an objective lens is led more to the front monitor.